Abstract of the Parent Grant: The extracellular matrix (ECM) is a highly dynamic component of the developing skeleton and craniofacial structures. While the transcriptional control of ECM composition during skeletal and craniofacial development is intensively studied, the post-transcriptional control of the secreted components of the ECM and its regulators are relatively poorly understood. We have identified a novel class of regulator of secreted proteins, the first known secreted protein tyrosine kinase (VLK) that is essential for normal craniofacial development and endochondral bone formation. We find that VLK phosphorylates a wide range of secreted proteins with established roles in skeletal development and maintenance of bone mass. VLK also phosphorylates resident ER proteins with specific roles in bone and cartilage ECM secretion, and thus may modify secreted proteins not only directly, but also indirectly through control of the secretion process in bone, tooth, and cartilage forming cells. In this proposal, we plan to establish the functional role of VLK-mediated phosphorylations during endochondral ossification, adult bone homeostasis and during fracture repair. We will use a combination of in vitro and in vivo approaches to identify substrates for VLK in skeletal progenitor cells, chondrocytes and osteoblasts, identify spatial and temporal patterns of VLK mediated phosphorylations in vivo, establish how VLK modifies chondrocyte differentiation and endochondral ossification, and examine the function of VLK phosphorylation of specific secreted substrates. These studies will define a novel mechanism for the regulation of ECM function in bone, providing a new approach to the therapeutic modulation of skeletal development, homeostasis, and repair.